There is a demand to reduce the rolling resistance of pneumatic tires in order to enhance the fuel economy performance of vehicles. In addition, simultaneously achieving excellent wet grip performance is necessary for the pneumatic tires. Silica is typically blended into a rubber composition for a tire in order to enhance low rolling resistance and wet grip performance. In this case, the compounded amount of silica should be increased to enhance the wet grip performance of the rubber composition. However, when the amount of silica is increased, there has been a problem in that the dispersibility of the silica is deteriorated to increase the rolling resistance. That is, with a rubber composition having a large compounded amount of silica, it has been difficult to achieve both low rolling resistance and wet grip performance.
On the other hand, tan δ at 60° C. of a rubber composition is often used as an indicator of rolling resistance. However, tan δ is a ratio of the storage elastic modulus, which is the component of the energy generated in a substance due to external force and strain that is stored inside the substance, and the loss elastic modulus, which is the component diffused to the outside, and tan δ does not essentially explain the factors that influence the magnitudes of these components. In recent years, ultra-small-angle X-ray scattering (USAXS) has been proposed as a technique for investigating the hierarchical structure of a rubber composition containing silica (primary particles, aggregates, agglomerates) (for example, see Non-Patent Document 1). However, the relationship between the hierarchical structure of the rubber composition containing silica and the rolling resistance and wet grip performance thereof has not yet been elucidated.